In its native Republic of Ireland, Saint Patrick’s Day is a public holiday, where parades, festivals, and céilithe are common sights all through March 17. It’s also a holiday in neighbouring Northern Ireland, as well as in regions as far-flung as Newfoundland and Labrador, Canada, and the British Overseas Territory of Montserrat. Across the Commonwealth, similar celebrations take hold every year—especially in areas with high concentrations of Irish expats.

The real Saint Patrick, a Romano-British Christian missionary, is only one of the patron saints of Ireland. There are also Brigit of Kildare—who some scholars believe may actually be a “Christianisation” of the pre-Christian Ireland God of the same name—and Columba, who spent much of his life spreading Christianity in Scotland, one of his other patronages.

Green, representing Gaelic Irish tradition, is the national colour of Ireland and commonly associated with Saint Patrick’s Day… but this isn’t actually Saint Patrick’s colour. Saint Patrick’s blue, of which there are many hues and shades, is based on the blue robes worn in Irish mythology by the sovereignty of Ireland herself.

The tradition of a green Saint Patrick’s Day began earlier, though—back in the 1640s. The “wearing of the green” actually comes from a song of the same name. It laments the persecution of United Irishmen, a group that rebelled against British rule in 1798, for wearing green.

Saint Patrick’s Day falls on what is said to be Patrick’s date of death some time in the late fifth century. According to his Confessio, he served as a slave for six years following his capture by Irish pirates at the age of 16. He soon after became a cleric, and later still a bishop. Not much else is known about his life beyond this.

However, that hasn’t stopped legends being attributed to him. The famous adage that he “banished the snakes” from Ireland has been widely thought to be just that—an adage—since snakes are unlikely to have ever been in Ireland at all. After all, it would be difficult for a grass snake to swim the length of the Irish Sea, no matter how good the beer.

A legend that likely has some bearing in truth, however, is that of Patrick’s shamrock—a symbol now widely associated with Ireland and Saint Patrick’s Day, as well as with good luck. He is said to have used the three-leaved clover to illustrate “three persons in one God”, a concept tied to more pagan beliefs popular in Ireland at the time.

Saint Patrick’s Day wasn’t actually a recognised holiday in Ireland until 1903, though it was a Catholic feast day for hundreds of years prior. It was celebrated with the first ever Saint Patrick’s Day parade in the town of Waterford.

Between 1927 and 1961, the sale of alcohol on March 17 was banned by the Irish Free State—perhaps a little ironic given the holiday’s associations now.